In case you plan to use an object oriented approach and wrap your C API code in classes, a great help is Notes Hound from http://www.noteshound.com. As we all know classes is something added as an afterthought in Domino Designer without any means of decently finding properties and methods and navigating them. I use this tool regularly to navigate in classes no matter which design element they are in; it works both for R5 or ND6. It feels like I'm doing Domino work in VB environment (just joking;-)

It is true swiss knife of Domino Developer, and the scoop is — this tool has been written by calling both COM and C APIs from VB; an excellent case of extreme coding. A pitty it's not an open source.